According to the state of the art, in outdoor computers, for example, wristop computers, equipped with barometers, weather alarms are used by indicating rapid variations in pressure.
A drawback of the state of the art is that variations in pressure are also caused by changes in the altitude of the user of the device. Thus, in order to avoid unnecessary alarms, it has been possible to use the weather alarm only when the user remains at a constant altitude. Thus, during a hike over ground with a varying elevation profile, devices according to the prior art would give many false alarms, because the changes in atmospheric pressure due to vertical movements are typically so much greater than the differences in atmospheric pressure caused by weather changes that a change in weather will be obscured beneath the pressure changes due to altitude. The alarm condition will be thus met due to a change in altitude. In practice, it has not been possible to implement a device giving a good weather alarm by applying existing knowledge.
Various methods for measuring pressure, based on calibrating an altitude definition with the aid of map data or GPS data, are known from U.S. Pat. Nos. 6,529,827, 6,434,485, and 6,381,540.
The GPS system is mainly intended for positioning that takes place at ground level, in a two-dimensional space. However, if the GPS device can contact at least four satellites, altitude data can also be defined, in among other ways, the manners disclosed in the aforementioned U.S. patents. Because altitude data (vertical data) is not actually a basic property of the OPS system, the definition of a vertical position using the GPS system is both slow and inaccurate, compared to the same system's accuracy and speed of definition of horizontal data.